(1) Field of the Invention:
This invention relates to a plastic lens resin having a high refractive index (hereinafter may also be called "refractivity"), low dispersion, and excellent heat resistance.
(2) Description of the Prior Art:
Plastic lenses have been finding utility as eye lenses and camera lenses and in optical devices in recent years since they are lighter, less fragile and dyeable compared with inorganic lenses.
As a resin employed widely for these purposes, there is a resin obtained by subjecting diethylene glycol bisallylcarbonate to radical polymerization. Its refractive index is however small (n.sub.D =1.50) compared with inorganic lenses (n.sub.D =1.52). In order to achieve optical characteristics equivalent to the inorganic lenses, it is necessary to increase the curvature of a lens so that the central thickness and peripheral thickness of the lens increase and the overall thickness of the lens hence becomes greater unavoidably. There is thus an outstanding demand for the development of a lens resin having a higher refractive index in order to overcome this inconvenience. It has therefore been proposed to introduce one or more aromatic rings, chlorine atoms or bromine atoms in the chemical structures of various monomers. The introduction of one or more aromatic rings alone can however hardly achieve a high refractive index, whereas the sole introduction of one or more halogen atoms results in a greater specific gravity and impairs the advantageous light weight feature of plastic lenses.
Since high refractivity, low dispersion and low specific gravity can be expected from the introduction of one or more sulfur atoms into the chemical structure of a monomer, sulfur-containing urethane resins have been proposed (Japanese Patent Laid-Open Nos. 194401/1985 and 217229/1985). These sulfur-containing urethane resins are however inferior in heat resistance to conventional radical-polymerization olefin resins, they are susceptible to deformations upon post processing such as lens dyeing and surface coating so that the processing temperature must be controlled low during such post processing. A limitation is hence imposed on their wide-spread use.